U.S. patent application number 16/167712 was filed with the patent office on 2019-02-14 for method for testing open phase detection system.
The applicant listed for this patent is Power Control Systems, Inc.. Invention is credited to Robert P. Bayly, Jeremy S. Davey, Michael L. McAnelly, Donald J. Norris.
Application Number | 20190049526 16/167712 |
Document ID | / |
Family ID | 64176667 |
Filed Date | 2019-02-14 |
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United States Patent
Application |
20190049526 |
Kind Code |
A1 |
McAnelly; Michael L. ; et
al. |
February 14, 2019 |
METHOD FOR TESTING OPEN PHASE DETECTION SYSTEM
Abstract
A high accuracy open phase detection system for power
transformers that uses a combination of logic controllers and
current transformers to recognize an open phase condition
experienced by the power transformers under no load, light load,
and full load conditions. A current to voltage and current to
current transformer on each phase are employed to detect the
excitation current and load current conditions. During an open
phase condition, a microprocessor detecting device, connected to
the current to voltage and current to current transformers,
monitors the appropriate power system quantities to determine the
existence of one or more open phase(s) with or without a ground or
an open phase with line charging capacitance. Through the
microprocessor monitor, the microprocessor detecting device can
alert operators to the loss of phase or abnormal conditions in the
power source. The data used by the microprocessors can be used to
calculate the magnitude and phase angle of the current in the power
source and detect abnormal system conditions. This invention also
employs a unique circuitry configuration to reduce the effects of
ambient noise on the open phase detection system.
Inventors: |
McAnelly; Michael L.; (Baton
Rouge, LA) ; Davey; Jeremy S.; (Denham Springs,
LA) ; Norris; Donald J.; (Prairieville, LA) ;
Bayly; Robert P.; (Baton Rouge, LA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Power Control Systems, Inc. |
Baton Rouge |
LA |
US |
|
|
Family ID: |
64176667 |
Appl. No.: |
16/167712 |
Filed: |
October 23, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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14262267 |
Apr 25, 2014 |
10132875 |
|
|
16167712 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02H 5/10 20130101; G01R
31/62 20200101; H02H 1/0007 20130101 |
International
Class: |
G01R 31/40 20060101
G01R031/40; H02H 1/00 20060101 H02H001/00; H02H 9/00 20060101
H02H009/00 |
Claims
1. A method for testing a device for detecting open phase
conditions of a power system transformer, comprising: a. adding an
isolated test winding through a current to voltage transformer and
a current to current transformers; and b. connecting said current
to voltage transformer and current to current transformer test
winding to a relay panel location within the device's relay
cabinet.
2. The method of claim 1 wherein said relay cabinet is located in a
safe environment for testing.
3. The method of claim 1, wherein said additional transformer
winding is manufactured within the moldings of the current to
voltage transformer and the current to current transformer.
4. The method of claim 1, wherein said test winding will be
polarity marked in the same manner as the primary and secondary
winding of the current to voltage and current to current
transformers.
5. The method of claim 1, wherein the device for detecting open
phase conditions of a power system transformer comprises: a. an
excitation monitoring sub-system for detecting an open phase when
said power system transformer is unloaded or partially loaded, with
a current at an amount of at least one-tenth percent of the power
system transformer's load, said excitation monitoring sub-system
comprising: i. an excitation monitoring current to voltage
transformer; and ii. a microprocessor voltage measuring device; b.
a load monitoring sub-system for detecting an open phase when said
power system transformer is loaded, with currents as high as fault
current in the 10,000 A range, said load monitoring sub-system
comprising: i. a load monitoring current to current transformer on
each phase; and ii. a microprocessor current measuring device; and
wherein said excitation monitoring current to voltage transformer
and said load monitoring current to current transformer are
magnetically coupled for direct measurement of the power system
transformer's primary currents.
6. A method for testing a metering system, a control system, a
current to current transformer used in a protective relay system, a
current to voltage transformer used in a protective relay system,
or other current monitoring device, said method comprising: a.
adding an isolated test winding through a current to voltage
transformer and a current to current transformers; and b.
connecting said current to voltage transformer and current to
current transformer test winding to a relay panel location within
the device's relay cabinet.
7. The method of claim 6 wherein said relay cabinet is located in a
safe environment for testing.
8. The method of claim 6, wherein said additional transformer
winding is manufactured within the moldings of the current to
voltage transformer and the current to current transformer.
9. The method of claim 6, wherein said test winding will be
polarity marked in the same manner as the primary and secondary
winding of the current to voltage and current to current
transformers.
10. The method of claim 6, wherein the device for detecting open
phase conditions of a power system transformer comprises: a. an
excitation monitoring sub-system for detecting an open phase when
said power system transformer is unloaded or partially loaded, with
a current at an amount of at least one-tenth percent of the power
system transformer's load, said excitation monitoring sub-system
comprising: i. an excitation monitoring current to voltage
transformer; and ii. a microprocessor voltage measuring device; b.
a load monitoring sub-system for detecting an open phase when said
power system transformer is loaded, with currents as high as fault
current in the 10,000 A range, said load monitoring sub-system
comprising: i. a load monitoring current to current transformer on
each phase; and ii. a microprocessor current measuring device; and
wherein said excitation monitoring current to voltage transformer
and said load monitoring current to current transformer are
magnetically coupled for direct measurement of the power system
transformer's primary currents.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Patent Application No. 61/839,501 filed on Jun. 26, 2013, U.S.
Provisional Patent Application No. 61/815,873 filed on Apr. 25,
2013. This application is a division of U.S. Non-Provisional patent
application Ser. No. 14/262,267 filed on Apr. 25, 2014. The
disclosures of the referenced applications are hereby incorporated
herein in their entirety by reference.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to the field of detecting the
various open phase and/or faulted conditions of a power source to
ensure the integrity of offsite, backup power sources for nuclear
power plants.
[0003] Since the implementation of nuclear power facilities, there
has been a strong concern regarding the potential hazard of release
of radioactive materials from facilities generating electricity
with nuclear power. Nuclear power safety depends on intelligent
planning, proper design of backup systems, high-quality components,
and a well-developed culture of safety in operations. The backup
systems in nuclear power plants are generally composed of offsite
transformers and onsite emergency generators, and these systems can
be used to power the emergency operations of a nuclear power plant
when the onsite power source is shut down, known as "Reactor
SCRAM." United States nuclear power plants are designed to cope
with Reactor SCRAM by having multiple backup power sources at the
ready. The Nuclear Regulatory Commission requires United States
nuclear power plants to demonstrate they can handle such situations
in order to meet NRC licensing requirements and legally remain in
operation.
[0004] The offsite power systems are typically referred to as
Reserve Station Service power transformers, Startup/Standby
transformers, or Start-Up power transformers. When there is a loss
of onsite power or an abnormal condition at the facility, the power
source of the critical safety systems is automatically transferred
to the offsite Reserve Station Service power transformers
(hereinafter the "RSS transformers"). If there is a loss of power
from the RSS transformer, emergency backup diesel generators begin
operating to provide electrical power to the plant safety
systems.
[0005] Given the critical task that the back-up and emergency power
sources serve, there has been increased concern regarding their
reliability. Open phase conditions in the back-up power systems
have presented a particularly troublesome problem, as the open
phase condition in the system has historically been difficult to
detect with existing technology.
[0006] The challenge that comes with detecting and remedying open
phase conditions of the backup power source is the difficulty of
measuring the low-level transformer excitation currents. The power
transformers used for Reserve Station Service duty in power plants
are typically unloaded when the plant is in normal operation and
draw only excitation current. These transformers are fed from the
high voltage power grid and are small in capacity when compared to
the available capacity of the high voltage power grid. Transformer
excitation current is typically less than 0.1% of the power
transformer's full load capacity. It is very difficult to measure
quantities of this value for power transformers connected to high
voltage systems because accurate direct measurement is a near
impossible task. Modern, state-of-the-art sensing devices used in
relay systems fed from the typical bushing current transformers are
not capable of recognizing the wide variations in conditions, for
example, the range of 0.08 amps to 22,000 amps. Measurements have
to be performed using current transformers to isolate the measuring
devices from the dangerous, high voltage circuits which operate as
high as 500,000 volts. These measuring devices are typically low
voltage and low power consuming devices that operate in the range
of several volts. The measuring devices require connection to the
current transformers to monitor quantities of the high voltage
system. However, those measuring devices' accuracy often suffer due
to ambient noise that is present in the high voltage system. Prior
to the disclosed method and device, there were no methods for
detecting open phase conditions on energized, unloaded or lightly
loaded transformers.
SUMMARY OF THE INVENTION
[0007] The present invention addresses all of the problems of prior
technologies. This invention utilizes a unique arrangement of logic
controllers and current sensors that are able to recognize the
open-phase condition. High impedance voltage measuring devices are
used to measure lower level currents, which is necessary for the
measurement of power transformer excitation current.
[0008] The device and method is comprised of current sensors which
are connected to the power source for detection of open phase and
abnormal conditions. Each current sensor is comprised of one
current to voltage transformer (hereinafter "CV transformer") and
one current to current transformer (hereinafter "CT transformer").
The CV transformer is used for monitoring the power source during
unloaded and energized conditions. The CT transformer is used for
monitoring the power source during loaded conditions. The current
sensors are placed on each phase of the high side winding of the
power source that is being monitored for open phase or abnormal
conditions. Each CV transformer has its full secondary winding
leads connected to the high impedance voltage measuring device
during energized power transformer no load conditions. The
invention utilizes high impedance devices for measuring the voltage
on the secondary windings of the CV transformers. This data is used
to determine, for the power source, the power transformer
excitation current, loss of transformer excitation current, phase
angle of the power transformer excitation current, and abnormal
system conditions. Abnormal system conditions include: loss of a
single phase, loss of multiple phases, loss of phase with a
short-circuit connected to the power transformer line, open phase
with line charging capacitance, open jumper inside of the power
transformer, open tap-changer connection in the power transformer,
open switch contact, open breaker contact, and other similar
conditions with the power source.
[0009] The open phase detection system is further comprised of a
novel arrangement of microprocessor based components with specially
designed sensors and associated circuitry. The system also includes
power sensing devices that can measure the wide range of power
system quantities. The sensors are specially designed to have the
ability to measure and operate at all levels, including low level
excitation current, lightly loaded conditions, full load current,
and current under various faulted conditions. The microprocessors
are programmed with an algorithm that smoothes the measured current
and compares the measured current to normal operation quantities to
determine the existence of abnormal conditions. The sensors also
have the ability to withstand the high levels of fault current and
are configured to provide maximum noise immunity. The secondary
winding of the sensors are connected in a balanced circuit
configuration, with a grounded center tap, and use shielded cable
to minimize ambient noise from common-mode coupling.
[0010] The system is capable of communication through the
microprocessors in order to convey information about the power
system and the open phase detection system to the control room. The
system uses communication media to the control room for
annunciation purposes. The system is also equipped with data
logging for tracking operating quantities for comparison with power
system disturbances for evaluation of correct operation. This also
includes time-synchronization with a self-contained satellite
clock. The system will be capable of tripping source
circuit-breakers when tripping is required. The system also has
appropriate test switches and disconnect switches to provide the
ability to test and isolate in a safe, reliable manner.
[0011] The system incorporates special noise immunity features to
facilitate consistent, correct operation in the noisy, high-voltage
substation environment. A benefit of the open phase detection
system is that it achieves correct detection of open phase
conditions on power transformers drawing only excitation current.
This feature is unique because it performs this function in an
environment where ambient noise in the power system is very high in
relation to the very small excitation current of a transformer.
[0012] In the event of an open phase condition, these components
work together to alert the operators of the open phase condition.
The current sensors transmit the magnitude and phase angle of each
phase to the microprocessors. The microprocessors will evaluate the
current magnitude and phase angle to determine whether an open
phase or other abnormal conditions exists on the power system. If
such a condition exists, the microprocessor communication system
will then alert the system operators to the open phase or abnormal
condition.
[0013] This invention also utilizes additional windings that are
molded into the CV transformers and CT transformers in order to
provide users the ability to use precise primary injection testing
in order to test the entire open phase detection system.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a single-line diagram of the circuit configuration
of the open phase detection system.
[0015] FIG. 2 is a single-line drawing of the circuit configuration
of the open phase detection system with additional components
connected, demonstrating additional embodiments.
[0016] FIG. 3 is a single-line drawing of the DC control circuit
that is connected to the microprocessor volt measuring device of
the excitation circuit in the open phase detection system.
DETAILED DESCRIPTION OF THE INVENTION
[0017] The method and apparatus disclosed herein is an open phase
detection system that detects open phase conditions on unloaded and
loaded auxiliary power systems and offsite power sources. For the
preferred embodiment, the offsite power sources are the RSS
transformers and the associated power supply system. Additionally,
this system detects open phase with faulted conditions on the
transformer side of the open phase. The system is capable of
monitoring, alarming, and tripping as required for specific plant
application.
[0018] The system utilizes current sensors, which consists of two
instrument transformers. These instrument transformers are window
type transformers that are completely electrically isolated from
the power conductors. This type of instrument transformer is
magnetically coupled and not physically connected to the power
system conductors such that the instrument transformer does not
introduce an additional point of failure to the offsite power
source. The first is a high ratio, high burden capacity CV
transformer that is used for the measurement of the excitation
current of each phase of the RSS transformer. The second is a low
ratio, low burden capacity CT transformer that is used to measure
the detection of open phase conditions when the RSS transformer is
loaded. These CV and CT transformers are placed in the most
convenient location for measuring the current in each phase of the
high side winding of the RSS transformer. These CV and CT
transformers may be contained in a single structure or separate
structures as necessary for the application. Space limitations and
obstructions will dictate the structure configuration and physical
dimensions.
[0019] The CV transformer on each phase is used to monitor the
offsite power source in the energized, unloaded mode, and each
phase shall have the secondary winding leads connected to the
measuring device. The CV transformer secondary winding will be a
high ratio winding specifically designed for monitoring the RSS
transformer excitation current. The secondary winding of this CV
transformer is center tapped and connected to ground to provide
connection in a balanced configuration and to provide common-mode
noise rejection. The CT transformer is a multi-ratio transformer
with the appropriate ratios and taps to closely match the RSS
transformer at full-load current and to accomplish measurement
without saturation or overload. In order to properly measure both
the excitation current and the full-load current, the CV and CT
transformers are connected around the primary conductors between
the offsite power source breaker and the Wye-point of the RSS
transformer. The devices can be mounted around the base of the
transformer high voltage bushings of the high side winding or
around the base of each phase of the high side neutral bushings if
available. For a Delta connected high side winding, the CV and CT
transformers are mounted on each phase between the source breaker
and on or around the high side bushing. The primary winding of the
CV and CT transformers is the high voltage conductor of the power
system. In some applications, an additional CT transformer will be
required on the RSS transformer between the Wye point and the
system ground for measurement of power transformer neutral current.
This device is necessary to protect the transformer in the event of
an open and grounded condition. For transformers with a Delta high
side connection, this additional CT transformer would not be
applicable.
[0020] Ambient noise in power systems is extremely high; therefore,
the sensors and microprocessor are specially designed and
configured to mitigate the effects of noise in the system so that
the small excitation current can be detected. By center-tapping the
CV transformers, a balanced configuration is achieved which helps
improve signal to noise ratio. Twisted, shielded pairs of wire also
can be used to provide further improvement of the signal to noise
ratio in the substation. Lead lengths from the CV and CT
transformers to microprocessor location are preferably limited to
100 feet to limit the effects of noise in a high voltage
environment.
[0021] The RSS transformer makes use of CV and CT transformers on
each phase. Each CV transformer is used as the excitation current
sensor and each CT transformer is used as the load current sensor.
The excitation and load sensors are specially designed
electromagnetic sensors. The sensors can be concentric and are
sized for the appropriate application based upon the capacities of
the offsite power source presently on the system. The sensors are
also designed to meet the dimensional requirements for installation
on the RSS transformer. The components inside of the excitation and
load sensors consist of a laminated, magnetic core material and
secondary windings typically made of copper. The core material and
windings can vary to match the excitation current, load currents,
and physical dimensions. The sensor material will commonly be
comprised of copper and iron; however, based upon the size,
voltage, and features of the transformer and additional conditions
of the application, other materials may be used. The sensors
themselves will typically use a combination of materials, such as
the suggested copper and iron, and the ratio between the two
materials is optimized based upon the size, voltage, and other
conditions of the application.
[0022] In order to construct the proper windings for the excitation
sensor and the load sensor, one skilled in the art would first
determine the excitation current and load current, plus the
operating range of those currents, as set by the rating of the
transformer. Generally, this information can be obtained through a
transformer test report. A person skilled in the art would, based
upon the current information contained in the test report, be able
to construct the proper winding ratio needed for this
application.
[0023] If desired, the excitation sensor and the load sensor can be
placed on the bushings of the RSS transformer, so as to not
encroach on the existing system.
[0024] As seen in FIG. 1, the system is comprised of the excitation
monitoring sub-system 1 and the load monitoring sub-system 2. The
device is connected in series between the offsite power source
breaker 9 and the Wye-point of the offsite power source 8. For the
explained example embodiment, the offsite power source 8 is an RSS
transformer. In the excitation monitoring sub-system 1, the
excitation monitoring CV transformer 3 is connected in parallel
with the microprocessor voltage measuring device 5. In the load
monitoring sub-system 2, the load monitoring CT transformer 4 is
connected in series with the microprocessor current measuring
device 6. The microprocessor components include a hardware platform
with analog input circuitry for measurements, coded algorithms for
decision making, and inputs and outputs for monitoring, tripping,
and alarming.
[0025] Within the coded algorithm, the abnormal conditions are
identified, and the logical evaluations are made to alarm or trip
as required. Among others, these abnormal conditions could be an
open phase conductor to the RSS transformer or an open phase
condition within the RSS transformer itself. The microprocessor
devices are capable of detecting from current magnitudes and phase
angles the existence of open or open with grounded conditions on
the high side supply to the RSS transformer and, if required,
provide coordinated tripping with other optional system protective
devices. The microprocessors of the system use a specialized
algorithm to detect an open phase or abnormal condition on the RSS
transformer. In the system, the load monitoring CT transformer 6
detects the analog values of the magnitude and phase angle of the
load currents for each phase of the RSS transformer. The magnitude
and phase angle of each phase current and the zero sequence current
is measured. For each current measurement, the microprocessor
relay/device performs an analog to digital conversion for the
signals.
[0026] Once the conversion is performed, the microprocessor
algorithm calibrates the current data using a smoothing function.
Smoothing functions are often used in statistics and image
processing to capture important patterns in data, while leaving out
noise or other fine-scale structures and rapid phenomena. In the
smoothing function, the original points are reduced to prevent
rapid, high-frequency step changes from resulting in false output
operations. The smoothing function utilized in this application
adds a small portion of the actual measured current to the
historical, existing current once every processing interval. In the
preferred embodiment, 0.1% of the actual measured current is added
to 99.9% of the historical, existing current quantity to derive a
new historical current value. This ensures that, should the
measured current quantity change quickly, the historical current
value actually evaluated for abnormal conditions changes more
slowly. Time delays can also be applied at various points in the
algorithm to make it less susceptible to false output
operations.
[0027] The smoothing function provides calibrated quantities for
the magnitudes and phase angles for each phase current. The
calibrated values for each phase current are passed through
separate comparator gates in the algorithm. An open phase condition
is declared when historical current magnitude is below a certain
percentage of the nominal magnitude of a normal, no load excitation
condition and at least one of the other phases has a current
magnitude above a certain percentage of the nominal magnitude of a
normal, no load excitation current condition. In the preferred
embodiment, these percentages would be below 20% and above 70% of
the nominal no load excitation magnitude of a current condition,
respectively. If an open phase condition is detected, a logical 1
will be generated by the comparator gate and if the logical 1
exceeds a certain time delay, the algorithm will close the output
contact, indicating an open phase condition.
[0028] The algorithm will perform a second analysis to detect an
open phase condition when the open conductor is also grounded. The
zero sequence phase quantity (magnitude and phase angle) measured
is divided by the positive sequence quantity. If the dividend
produced exceeds a certain value, then an open phase with a
grounded condition is declared. The specific minimum dividend value
to assert an alarm is based upon the particular power transformer
and can be easily generated by one skilled in the art.
[0029] The section of the algorithm in the microprocessors
described above performs its function in the no load excitation
mode. However, if the value of the current exceeds a certain
amount, the algorithm switches off the excitation mode and the
current level will be high enough to be detectable by the load mode
section of the algorithm. In the preferred embodiment, that certain
amount is three times the no load excitation current value. In the
load mode, the zero sequence phase quantity (magnitude and phase
angle) measured is divided by the positive sequence quantity. If
the dividend produced exceeds a certain value, then an open phase
condition is declared by the algorithm. This state mirrors the
excitation mode method; so, under the load mode state, the
comparator gates will raise a logical 1 if the ratio of zero
sequence to positive sequence current is above a certain level.
[0030] The microprocessor monitors are equipped with communications
processors for trending data and for sequential event recording. An
integrated GPS clock is also used to provide synchronized
measurements of event data recorded in the microprocessor so that
it may be compared to other recording data on the transmission
system.
[0031] The microprocessor output is connected to an alarm circuit
that is used to alert the operators of an open phase condition. The
alarm circuit 11 is depicted in FIG. 3. Control power 22 is
preferably supplied to the microprocessors and control relays from
a secure power source located in the power plant such as, for
example, a DC powered battery. In the alarm circuit 11, the
microprocessor output 23 is connected in series to a relay 12 to
short circuit the excitation monitoring instrument transformer 3.
The microprocessor has multiple outputs 24 and 14 that are used for
tripping purposes, power plant control room annunciation purposes,
and local annunciation at the relay cabinet. When the
microprocessor detects an open phase or abnormal condition on the
power source, the microprocessor will send the signal for output 14
to close. This will operate the local annunciator 15 and control
room annunciator 16. The control room annunciator 16 and the local
annunciator 15 provide indication of the various open phase
conditions.
[0032] During an open phase condition, the excitation monitoring
sub-system 1 through the microprocessor voltage monitoring device 5
makes decisions based on the microprocessor algorithm to alarm or
trip for the various open phase conditions in the excitation mode.
The microprocessor current measuring device 6 monitors the
conditions of the phases and performs calculations to determine the
magnitude and phase angle of the abnormal conditions by the
algorithm. Once it has made the determination that a phase is open,
the microprocessor system will alert the operators via annunciation
panels 15 and 16 that a phase is open. Based upon the signals and
readings provided by the open phase detection system, plant
operations personnel can take appropriate mitigating action. In
some cases, automatic tripping of the power source may be required
depending on the configuration of the individual plant
application.
[0033] In FIG. 2, additional optional components are connected to
protect the open phase detection system. Depending on the needs of
the open phase detection system, surge protection can be used to
protect equipment and personnel from the high transient conditions
that could be introduced from the high voltage power system and its
reactive components. In one embodiment, a surge protection device
10 is connected in parallel to the microprocessor voltage measuring
device 5.
[0034] For an RSS transformer that is connected in a Wye
configuration on the high side of the transformer, an additional,
zero sequence instrument transformer will be required. This zero
sequence instrument transformer 20 is a CT transformer and is
connected to the offsite power source 8 between the offsite power
source's Wye point and the system ground. This zero sequence
instrument transformer 20 is used to measure any transformer
neutral current. This device is necessary to protect the RSS
transformer in the event of an open and grounded condition. For
transformers with a Delta high side connection, the zero sequence
instrument transformer 20 would not be applicable nor
necessary.
[0035] If zero sequence instrument transformer 20 has been
included, the comparator gates in the algorithm will also test for
abnormal conditions measured by the zero sequence instrument
transformer 20. The microprocessor algorithm will detect a zero
sequence fault scenario when there is an open phase condition or
ground fault detected with the zero sequence instrument
transformer. In this case, the algorithm would be analyzing the
magnitude of the current from the zero sequence instrument
transformer. The comparator gate will raise a logical 1, or output
alarm condition, if the value of the output to the microprocessor
exceeds a preset value, indicating a phase-to-ground fault or an
open phase condition on the electrical source. The preset maximum
value is based upon the particular transformer and can be easily
generated by one skilled in the art.
[0036] In some applications, the current levels during load
conditions could damage the excitation monitoring sub-system
components. Therefore, some applications may require a switching
device, such as an auxiliary relay actuated by the load current
measuring sub-system, to short-circuit the excitation current
monitoring instrument transformer to allow continuous operation
when the RSS transformer current level is above the no load
excitation level. This will depend on the particular design
characteristics of the CV transformer and its self-limiting
capabilities. In the preferred embodiment, the CV transformer will
be designed with surge and overvoltage protection to allow
continuous operation under load current conditions. As seen in FIG.
2, the short-circuiting relay 19, would be connected in parallel
with the microprocessor volt measuring device 5 and the excitation
sensor 3. The short-circuiting relay is powered by the control
power circuit 11. During an open phase condition where shorting
relay 19 has been included, once the current levels are above
minimum load levels, the short circuiting relay will short circuit
the excitation monitoring sub-system, protecting the circuit and
equipment.
[0037] In some applications, additional load impedance may be
implemented to optimally match the output impedance of the
excitation sensor to the input impedance of the microprocessor
based measuring equipment. In one embodiment, a load resistor 17
can be connected in parallel, as seen in FIG. 2, to perform this
function.
[0038] In another embodiment, additional components can be used to
enable a more accurate metering capability. In FIG. 2, an auxiliary
transformer 18 has been connected in parallel to step up the
voltage to enable more accurate metering capabilities.
[0039] In applications where over voltage of the excitation CV
transformer could occur, a saturable reactor could also be
connected in parallel with the excitation CV transformer in order
to limit voltage and prevent saturation of the CV transformer core.
In FIG. 2, a saturable reactor 25 has been connected in parallel
for this application. The voltage of the excitation CV transformer
is limited by the said saturable reactor to protect equipment and
personnel.
[0040] Some applications may require a method for short circuiting
the excitation monitoring instrument transformer upon loss of
control power. The loss of control power protective device 7 can be
seen in FIG. 2. An example of a loss of control power protective
device is an auxiliary relay.
[0041] In some cases, the output voltage of the excitation sensor
may not be high enough to provide reliable excitation monitoring.
Series-compensating capacitors can be added in series between the
output of the step up transformer and the voltage monitoring
element of the microprocessor to offset system reactance and
increase microprocessor voltage input. In FIG. 2,
series-compensating capacitors 21 have been added for this
application.
[0042] For all of the additional components added in FIG. 2, those
components can be duplicated to ensure system protection, safety,
and reliability.
[0043] To ensure the operability of the system, a means for
applying a test current to the primary winding of the CV and CT
transformers is necessary to allow personnel to perform testing
without having to access the dangerous, high voltage conductor
passing through the CV and CT transformer primary windows. This is
accomplished by adding a test winding 26 passing through the CV and
CT transformer windows. Test winding 26 is then wired to the
microprocessor relay location. This test winding 26 will allow for
connection of the relay as well as the CV and CT transformer
primary windings to a commercial testing device without leaving the
immediate proximity of the microprocessor relay.
[0044] The test winding is manufactured within the molding of the
CV and CT transformers and brought out to the CV and CT transformer
junction box where it is then wired to connect within the relay
cabinet of the open phase detection system. This test winding will
be polarity marked in the same manner as the primary and secondary
winding of the CV and CT transformers. This will allow complete
testing of the open phase detection system by primary injection.
Test systems like state simulators, used for testing protective
relays, can be used to test the entire open phase detection system
and the CV and CT transformers. Precise primary injection testing
can thus be performed.
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